White captured the pawn on c3 and got ahead in the development, since his piece can already move along the open lines. He will develop his pieces like this: Nf3, Bc4, Qe2, 0-0, Rd1, Bf4 (g5), Rc1 - depending on the moves of black, of course.

Sometimes white can attack with the pawn push to e5 and open the lines in front of the uncastled black king. Another typical attack moves the knight to b5 and puts pressure on the d6 and c7 square (i.e. in combination with Bf4 this is a real threat).

Black has various options, but some of them are just changes in move order. The most flexible move seems to be 4...Nc6, since it is a good developing move and prevents the pawn e4 from advancing. This pawn push is one of the attacking motives that white has, i.e. after 4...Nf65. e5.